Meet the Bacteria Devastating Olive Groves and Vineyards

Summary

Xylella fas­tidiosa is a bac­terium caus­ing plant dis­eases, includ­ing Olive Quick Decline Syndrome (OQDS), with an annual eco­nomic impact of over €5.5 bil­lion. The bac­terium can spread through plants via the xylem, lead­ing to water stress and defi­cien­cies in ele­ments, caus­ing symp­toms of dis­eases, and efforts to con­trol it focus on pre­ven­tion, con­tain­ment, and research into treat­ment meth­ods.

One of the European Union’s top 20 pri­or­ity plant pests, Xylella fas­tidiosa is a bac­terium that causes a vari­ety of plant dis­eases. 

It causes the deadly Olive Quick Decline Syndrome (OQDS), which has led to wide­spread out­breaks in Europe over the past 15 years, and is esti­mated to have an annual eco­nomic impact of more than €5.5 bil­lion.

The ori­gins of the bac­terium in Europe and glob­ally

Xylella fas­tidiosa is one of only two known species of Xylella; the other is Xylella tai­wa­nen­sis, which causes pear leaf scorch in Asian pears on the island of Taiwan.

An aer­o­bic, Gram-neg­a­tive bac­terium that grows in the water trans­port tis­sues of plants (xylem), X. fas­tidiosa is known to cause numer­ous plant dis­eases world­wide. 

The bac­te­ria can travel freely through plants via the xylem, con­stantly mul­ti­ply­ing as they do so. 

Once their num­bers reach a crit­i­cal level, the result­ing biofilm blocks the xylem, lead­ing to water stress and defi­cien­cies in ele­ments such as zinc and iron, which cause many of the symp­toms asso­ci­ated with the dis­eases the pathogen is linked to.

The first reports of such a dis­ease occurred in 1892 when an unknown plague wiped out approx­i­mately 14,000 hectares (34,600 acres) of California vine­yards. 

This ​“Anaheim dis­ease” was later named Pierce’s dis­ease after Newton Pierce, the bac­te­ri­ol­o­gist brought in to study the out­break. 

Pierce cor­rectly sur­mised that a micro­scopic infec­tious agent caused the dis­ease, although he was unable to iso­late or iden­tify the spe­cific agent.

Assumed to be a virus for most of the 20^th cen­tury, it was not until 1973 that X. Fastidiosa was rec­og­nized as a bac­terium. It was not until 1987 that the bac­terium was for­mally described and named Xylella fas­tidiosa by Wells et al. 

Since then, 696 plant species from 88 botan­i­cal fam­i­lies have been iden­ti­fied as suit­able hosts for the pathogen.

Among the dis­eases known to be caused by Xylella are sev­eral of sig­nif­i­cant agri­cul­tural and eco­nomic impor­tance. These include the afore­men­tioned Pierce’s dis­ease, which cur­rently causes the California wine-mak­ing indus­try esti­mated annual losses of $104 (€92) mil­lion, olive leaf scorch and OQDS.

OQDS causes with­er­ing and des­ic­ca­tion of olive leaves, twigs and branches, pre­vent­ing the trees from bear­ing fruit and even­tu­ally lead­ing to the col­lapse and death of the tree.

Worst-case pre­dic­tive mod­els show total eco­nomic losses of up to €5.6 bil­lion in Italy alone by 2070, and an esti­mated 100,000 jobs have already been lost due to out­breaks in the coun­try.

Due to its destruc­tive effects and its abil­ity to rapidly adapt to new envi­ron­ments and hosts, Xylella fas­tidiosa is reg­u­lated in the E.U. as a quar­an­tine organ­ism. Its intro­duc­tion into, and move­ment within, the union ter­ri­tory is pro­hib­ited by law.

How Xylella spreads and where it is cur­rently found

Native to Central America, Xylella fas­tidiosa is trans­mit­ted between host plants by xylem-feed­ing insects from the Cicadellidae (leafhop­per) and Cercopidae (spit­tle­bug and froghop­per) fam­i­lies. 

Such insects are capa­ble of only rudi­men­tary flight over short dis­tances (about 100 meters), but have been recorded trav­el­ing much longer dis­tances when car­ried by the wind. Bacterial trans­fer has also been shown to occur below ground via root grafts.

Long-dis­tance spread most often occurs through the move­ment of infected plants. This is believed to be how the pathogen was intro­duced to Italy and other European nations.

In October 2013, Xylella fas­tidiosa was found infect­ing olive trees in the region of Puglia in south­ern Italy. 

This was the first time the bac­terium had been reported within the European Union. The dis­ease caused a rapid decline in olive grove yields, and by April 2015, it was affect­ing the whole province of Lecce and other zones of Puglia.

The sub­species involved in Italy has been iden­ti­fied as X. fas­tidiosa subsp. pauca, a strain that shows a marked pref­er­ence for olive trees and warm cli­mates. This sub­species has since been listed under the Agricultural Bioterrorism Protection Act in the United States because of its dev­as­tat­ing poten­tial.

In response to the Italian out­breaks, the European Food Safety Authority (EFSA) con­vened an extra­or­di­nary sci­en­tific work­shop in November 2015. 

More than 100 sci­en­tists from around the world attended the event to iden­tify major knowl­edge gaps and dis­cuss research pri­or­i­ties regard­ing the pathogen. 

During the same month, the EFSA con­cluded from ongo­ing exper­i­ments in Puglia that grapevines were a pos­si­ble reser­voir of Xylella in the region.

By October 2015, the pathogen had reached Provence-Alpes-Côte d’Azur on the main­land of France, where the sub­species X. fas­tidiosa subsp. mul­ti­plex was found to have infected myr­tle-leaf milk­wort, a plant species intro­duced from South Africa. 

The fol­low­ing year, the bac­terium was iden­ti­fied in Corsica and Germany. In 2017, it was detected on the Spanish islands of Mallorca and Ibiza, and sub­se­quently on the Spanish main­land.

Xylella has since been found in olive trees and other host plants across the Iberian Peninsula, as well as in Lebanon and Israel in the Middle East.

The role of cli­mate change in Xylella’s spread

Substantial research indi­cates that cli­mate change increases the risk of plant dis­ease out­breaks, with changes in tem­per­a­ture and humid­ity being the pri­mary dri­vers.

As global tem­per­a­tures rise, the geo­graphic range of many pathogens expands, expos­ing new regions and plant species to dis­eases pre­vi­ously restricted to warmer cli­mates. 

Higher tem­per­a­tures are gen­er­ally con­ducive to the pro­lif­er­a­tion and prop­a­ga­tion of fun­gal and bac­te­r­ial species, espe­cially when com­bined with ele­vated humid­ity. 

Additionally, higher min­i­mum tem­per­a­tures extend the sea­son­ally active period of organ­isms and increase their abil­ity to sur­vive the win­ter and per­sist in the envi­ron­ment. This applies not only to pathogens but also to their vec­tors.

In addi­tion to favor­ing many pathogens, higher tem­per­a­tures can weaken a plan­t’s nat­ural defen­sive mech­a­nisms through processes such as heat and water stress, mak­ing them more vul­ner­a­ble to infec­tion and more likely to suf­fer greater dam­age and higher mor­tal­ity rates.

Specifically regard­ing Xylella fas­tidiosa, a recent cli­mate-dri­ven epi­demi­o­log­i­cal model ana­lyzed the vul­ner­a­bil­ity of European lands to the dis­ease in dif­fer­ent cli­mate change sce­nar­ios by assess­ing the cli­matic con­di­tions favored by both the pathogen and its pri­mary vec­tor, Philaenus spumar­ius, also known as the meadow froghop­per or meadow spit­tle­bug. This insect has pre­vi­ously been iden­ti­fied as the vec­tor respon­si­ble for spread­ing the bac­terium in Italian olive groves.

The study found that a global mean tem­per­a­ture rise of 1.5 °C increases the per­cent­age of total land area at risk in Europe to 0.32 per­cent, while a rise of 4 °C increases the area to 1.87 per­cent. 

Within the range of tem­per­a­ture increases ana­lyzed, a tip­ping point of a 3 °C increase was iden­ti­fied. Beyond this thresh­old, the researchers found that the risk of the pathogen spread­ing north of the Mediterranean region becomes remark­ably higher, allow­ing it to spread rapidly into pre­vi­ously unaf­fected areas.

The authors also assert that before the mid-1990s, European cli­matic con­di­tions, except those of the Mediterranean islands, most likely pre­vented the bac­terium from estab­lish­ing itself on the con­ti­nent.

Efforts to con­trol Xylella fas­tidiosa

As there is no known cure for dis­eased plants, cur­rent con­trol mea­sures focus on pre­ven­tion and con­tain­ment. 

The most effec­tive strat­egy in com­mon use requires both the com­pre­hen­sive removal of infected plant mat­ter, which can act as a reser­voir for the bac­terium, and the con­trol of insect vec­tor pop­u­la­tions.

In addi­tion to the com­plete removal of plant mat­ter known to be infected, the EFSA rec­om­mends cre­at­ing a ​“buffer zone” of at least 100 meters from which all sus­cep­ti­ble plant species are also removed and destroyed.

Due to the vir­u­lent nature of the pathogen, experts rec­om­mend using pro­tec­tive mea­sures when remov­ing and trans­port­ing all organic mate­r­ial dur­ing this process.

The process of con­trol­ling insect vec­tors is sim­i­larly involved, requir­ing not only the elim­i­na­tion of the organ­isms them­selves but also their habi­tats. 

This is nec­es­sary due to the polyphagous nature and multi-stage life­cy­cles of such insects. Philaenus spumar­ius, for exam­ple, is known to feed on at least 170 host plants and devel­ops through five sep­a­rate stages after hatch­ing.

Treatment and research for Xylella fas­tidiosa

Combinations of changes in crop­ping meth­ods, bac­te­ri­ci­dal treat­ments, and inter­ven­tions aimed at enhanc­ing the phys­i­o­log­i­cal state of the host have shown promise in impact­ing dis­ease devel­op­ment, even to the point of allow­ing har­vest­ing to resume. To date, how­ever, none have proven suc­cess­ful at erad­i­cat­ing the pathogen in an infected plant.

Research into treat­ment meth­ods is severely cur­tailed by Xylella’s quar­an­tine sta­tus, espe­cially within the E.U. Other EU restric­tions include the pro­hi­bi­tion on using antibi­otics for plant pro­tec­tion. Fields of research, there­fore, vary from one geo­graph­i­cal region to another.

In the United States, where antibi­otic use is autho­rized for use in plants, infor­ma­tion is avail­able from tri­als of antibi­otics such as oxyte­tra­cy­cline, tetra­cy­cline and strep­to­mycin in the foliar treat­ment of Pierce’s dis­ease and of microin­jec­tion of oxyte­tra­cy­cline in the treat­ment of Xylella-induced leaf scorch in American elm. 

Although such tri­als have demon­strated remis­sion of symp­toms, none have suc­ceeded in elim­i­nat­ing infec­tion, and symp­toms returned after treat­ment was stopped.

A major ini­tia­tive within Europe is the Biovexo Project, a Bio-Based Industries Joint Undertaking (BBI-JU) Innovation Action launched in 2020 under the European Union’s Horizon 2020 research and inno­va­tion pro­gram.

Aimed specif­i­cally at com­bat­ing Xylella in olive cul­ti­va­tion, BIOVEXO is devel­op­ing two main classes of envi­ron­men­tally-friendly biopes­ti­cides: ​“X‑biopesticides,” which tar­get the pathogen directly, and ​“V‑biopesticides,” which tar­get the spit­tle­bugs that act as the pathogen’s pri­mary trans­mis­sion vec­tor. 

The com­po­nent sub­stances being tri­aled are bac­te­r­ial strains, a micro­bial metabo­lite, plant extracts and an ento­mopath­o­genic fun­gus.

In a novel approach, recent research in Brazil involves N‑acetylcysteine, a com­mon mucolytic drug used to treat parac­eta­mol over­dose and to loosen thick mucus in human cases of dis­or­ders such as pneu­mo­nia and bron­chi­tis. 

While the mech­a­nisms respon­si­ble are not yet fully under­stood, ini­tial results have shown the effec­tive­ness of the drug in dis­rupt­ing bac­te­r­ial biofilms when applied by irri­ga­tion to hydro­ponic or field crops.

Given the role that biofilms play in pro­tect­ing bac­te­ria against antimi­cro­bial treat­ments and ulti­mately lead­ing to bac­te­r­ial resis­tance, this area of research may be on the rise, as break­ing down the pro­tec­tive biofilm matrix could sig­nif­i­cantly increase the effec­tive­ness of treat­ments tar­get­ing the Xylella bac­terium directly.

Until a means is found to accu­rately and sys­tem­at­i­cally kill the pathogen through­out its host, as this research sug­gests might one day be pos­si­ble, quar­an­tine and destruc­tion of infected plants will likely remain the sin­gle most effec­tive method of con­trol.